Evaluating Surface Mechanisms for Catalytic Combustion Of H2 and CH4 on Pd Catalysts

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Seyed-Reihani, Seyed-Abdolreza
Jackson, Gregory S
ABSTRACT Title of Dissertation: EVALUATING SURFACE MECHANISMS FOR CATALYTIC COMBUSTION OF H2 AND CH4 ON Pd CATALYSTS Seyed-Abdolreza Seyed-Reihani, Doctor of Philosophy, 2005 Dissertation directed by: Associate Professor Gregory Jackson Department of Mechanical Engineering Applications of fuel-lean catalytic combustion for power generation and exhaust heat recovery have raised the desire for reactor optimization. Such optimization requires adequately detailed surface chemistry models to predict reactor performance over a broad range of conditions relevant to the application. This study presents experimental studies in well-defined micro-reactors for catalytic combustion of CH4 and H2 on g-Al2O3 supported Pd catalysts, which are used to evaluate and refine surface chemistry mechanisms. The experimental results are compared to predictions by a transient numerical model for a catalytic channel flow with intra-phase diffusion in the porous washcoat support. Mechanisms for low temperature (< 250ºC) combustion of H2 under excess O2 and for relatively high temperature (> 400ºC) CH4 combustion under excess O2 have been developed and evaluated by comparison of experimental results in well-defined microreactors with transient model predictions. Low-temperature catalytic combustion over Pd-catalysts of very lean H2/O2 mixture diluted in N2 has been studied in the catalytic washcoat micro-reactor with transient exhaust monitoring using mass spectrometry. Experimental results reveal the important features of the Pd-H2-O2 surface chemistry under excess O2, particularly the effects of competitive adsorption/desorption of both the reactants and H2O product. Results show that H2 conversion depends on equivalence ratio at temperatures £ 125°C and on H2O vapor < 125°C. A proposed multi-step surface chemistry predicts based on detailed elementary reaction steps with thermodynamic reversibility and surface species interaction potentials captures the trends for conversion with respect to inlet temperature and water vapor. Intrinsic low dimensional manifolds (ILDM) were identified for the heterogeneous Pd-H2-O2 kinetics and the results show how specific species equilibration define the slowest modes in the catalytic reaction system. For the fuel lean CH4 oxidation over supported Pd catalysts, isothermal time-on-stream microreactor experiments and heating/cooling cyclic tests from 400°C to 850°C revealed the effects of PdO reduction/reoxidation on CH4 combustion kinetics. Test results with different H2O concentration revealed that competitive CH4 and H2O adsorption impacts on the activity only under catalyst conditions dominated by PdO. A detailed Pd-CH4-O2 surface mechanism predicts the impact of Pd reduction/reoxidation on CH4 oxidation rates. A post-process sensitivity analysis reveals the important reactions steps and provides a means for improving the detailed mechanism for predicting the complex hysteretic kinetics of CH4 oxidation on Pd.